S. Messina

Magnetic activity in the photosphere of CoRoT-Exo-2a - Active longitudes and short-term spot cycle in a young Sun-like star

Context. The space experiment CoRoT has recently detected transits by a hot Jupiter across the disk of an active G7V star (CoRoTExo-2a) that can be considered as a good proxy for the Sun at an age of approximately 0.5 Gyr. Aims. We present a spot modelling of the optical variability of the star during 142 days of uninterrupted observations performed by CoRoT with unprecedented photometric precision. Methods. We apply spot modelling approaches previously tested in the case of the Sun by modelling total solar irradiance variatio ns, a good proxy for the optical flux variations of the Sun as a star . The best results in terms of mapping of the surface brightness inhomogeneities are obtained by means of maximum entropy regularized models. To model the light curve of CoRoT-Exo-2a, we take into account both the photometric effects of cool spots as well as those of solar-like faculae, ado pting solar analogy. Results. Two active longitudes initially on opposite hemispheres are found on the photosphere of CoRoT-Exo-2a with a rotation period of 4.522± 0.024 days. Their separation changes by≈ 80 ◦ during the time span of the observations. From this variation, a relative amplitude of the surface differential rotation lower than ∼ 1 percent is estimated. Individual spots form within the act ive longitudes and show an angular velocity about∼ 1 percent smaller than that of the longitude pattern. The tot al spotted area shows a cyclic oscillation with a period of 28.9± 4.3 days, which is close to 10 times the synodic period of the planet as seen by the rotating active longitudes. We discuss the effects of solar-like faculae on our models, finding indication of a facular contribution to the optical flux variations of CoRoT-Exo-2a being significantly smaller than in the present Sun. Conclusions. The implications of such results for the internal rotation o f CoRoT-Exo-2a are discussed on the basis of solar analogy. A possible magnetic star-planet interaction is suggested by the cyclic variation of the spotted area. Alternatively, t he 28.9-d cycle may be related to Rossby-type waves propagating in the subphotospheric layers of the star.

Multiple and changing cycles of active stars II. Results

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Conclusions. Stellar activity cycles are generally multiple and variable.

RACE-OC project: Rotation and variability of young stellar associations within 100 pc

Context. Examining the angular momentum of stars and its interplay with their magnetic fields represent a promising way to probe the stellar internal structure and evolution of low-mass stars. Aims. We attempt to determine the rotational and magnetic-related activity properties of stars at different stages of evolution.We focused our attention primarily on members of clusters and young stellar associations of known ages. In this study, our targets are 6 young loose stellar associations within 100 pc and with ages in the range 8-70 Myr: TW Hydrae (~8 Myr), β Pictoris (~10 Myr), Tucana/Horologium, Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional rotational data for α Persei and the Pleiades from the literature are also considered. Methods. Rotational periods of stars exhibiting rotational modulation due to photospheric magnetic activity (i.e., starspots) were determined by applying the Lomb-Scargle periodogram technique to photometric time-series data obtained by the All Sky Automated Survey (ASAS). The magnetic activity level was derived from the amplitude of the V lightcurves. The statistical significance of the rotational evolution at different ages was inferred by applying a two-sided Kolmogorov-Smirnov test to subsequent age-bins. Results. We detected the rotational modulation and measured the rotation periods of 93 stars for the first time, and confirmed the periods of 41 stars already known from the literature. For an additional 10 stars, we revised the period determinations by other authors. The sample was augmented with periods of 21 additional stars retrieved from the literature. In this way, for the first time we were able to determine the largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, as well as increase by 150% the number of known periodic members of AB Dor. Conclusions. The analysis of the rotation periods in young stellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC 2264 data from the literature, has allowed us to find that in the 0.6-1.2 M ⊙ range the most significant variations in the rotation period distributipn are the spin-up between 9 and 30 Myr and the spin-down between 70 and 110 Myr. Variations of between 30 and 70 Myr are rather doubtful, despite the median period indicating a significant spin-up. The photospheric activity level is found to be correlated with rotation at ages greater than ~70 Myr and to show some additional age dependence besides that related to rotation and mass.

Dissecting the Moth: discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging

The debris disk known as “The Moth” is named after its unusually asymmetric surface brightness distribution. It is locat ed around the � 90 Myr old G8V star HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6� m. Polarimetric observations suggested that the circumstellar material consists of two d istinct components, a nearly edge-on disk or ring, and a swept-back feature, the result of interaction with the interstellar medium. We r esolve both components at unprecedented resolution with VLT/NACO Hband imaging. Using optimized angular differential imaging techniques to remove the light of the star, we reveal the disk component as a distinct narrow ring at inclination i = 84: 3�1: 0 � . We determine a semi-major axis of a = 61: 25�0: 85 AU and an eccentricity of e = 0: 045�0: 015, assuming that periastron is located along the apparent disk major axis. Therefore, the ring center is offset from the star by at least 2: 75�0: 85 AU. The offset, together with a relatively steep inner rim, could indic ate a planetary companion that perturbs the remnant planetesimal belt. From our imaging data we set upper mass limits for companions that exclude any object above the deuterium-burning limit for separations down to 0: 00 3. The ring shows a strong brightness asymmetry along both the major and minor axis. A brighter front side could indicate forward-scattering grains, while the brightness difference between the NE and SW components can be only partly explained by the ring center offset, suggesting additional density enhancements on one side of the ring. The swept-back component appears as two streamers originating near the NE and SW edges of the debris ring.

The GAPS programme with HARPS-N at TNG - I. Observations of the Rossiter-McLaughlin effect and characterisation of the transiting system Qatar-1

Context. Our understanding of the formation and evolution of planetary systems is still fragmentary because most of the current data provide limited information about the orbital structure and dynamics of these systems. The knowledge of the orbital properties for a variety of systems and at di erent ages yields information on planet migration and on star-planet tidal interaction mechanisms. Aims. In this context, a long-term, multi-purpose, observational programme has started with HARPS-N at TNG and aims to characterise the global architectural properties of exoplanetary systems. The goal of this first paper is to fully characterise the orbital properties of the transiting system Qatar-1 as well as the physical properties of the star and the planet. Methods. We exploit HARPS-N high-precision radial velocity measurements obtained during a transit to measure the Rossiter-McLaughlin e ect in the Qatar-1 system, and out-of-transit measurements to redetermine the spectroscopic orbit. New photometric-transit light-curves were analysed and a spectroscopic characterisation of the host star atmospheric parameters was performed based on various methods (line equivalent width ratios, spectral synthesis, spectral energy distribution). Results. We achieved a significant improvement in the accuracy of the orbital parameters and derived the spin-orbit alignment of the system; this information, combined with the spectroscopic determination of the host star properties (rotation, Te , logg, metallicity), allows us to derive the fundamental physical parameters for star and planet (masses and radii). The orbital solution for the Qatar-1 system is consistent with a circular orbit and the system presents a sky-projected obliquity of = 8:4 7:1 deg. The planet, with a mass of 1:33 0:05 MJ, is found to be significantly more massive than previously reported. The host star is confirmed to be metal-rich ([Fe/H] = 0:20 0:10) and slowly rotating (v sinI = 1:7 0:3 km s 1 ), though moderately active, as indicated by the strong chromospheric emission in the Caii H&K line cores (logR 0 4:60). Conclusions. We find that the system is well aligned and fits well within the general versus Te trend. We can definitely rule out any significant orbital eccentricity. The evolutionary status of the system is inferred based on gyrochronology, and the present orbital configuration and timescale for orbital decay are discussed in terms of star-planet tidal interactions.

Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars

The maximum amplitude ( A max ) of spot-induced brightness variations from long-term V -band photometry and the ratio L X / L bol between X-ray and bolometric luminosities are suitable indicators of the level of magnetic activity in the photosphere and in the corona of late-type stars, respectively. By using these activity indicators we investigate the dependence of coronal X-ray emission on the level of photospheric starspot activity in a homogeneous sample of low mass main sequence field and cluster stars of different ages (IC 2602, IC 4665, IC 2391, α Persei, Pleiades and Hyades). First, the activity-rotation connection at the photospheric level is re-analysed, as well as its dependence on spectral type and age. The upper envelope of

Photospheric activity, rotation, and radial velocity variations of the planet-hosting star CoRoT-7

A_{\rm max}

The REM telescope: detecting the near infra-red counterparts of Gamma-Ray Bursts and the prompt behavior of their optical continuum

increases monotonically with decreasing rotational period ( P ) and Rossby number ( R 0 ) showing a break around 1.1 d that separates two rotation regimes where the starspot activity shows different behaviours. The

Modelling the rotational evolution of solar-like stars: the rotational coupling time-scale

A_{\rm max}

RACE-OC project: rotation and variability in the ϵ Chamaeleontis, Octans, and Argus stellar associations

– P and A max – R 0 relations are fitted with linear, exponential and power laws to look for the function which best represents the trend of the data. The highest values of A max are found among K-type stars and at the ages of α Persei and Pleiades. We also analyse the activity-rotation connection at the coronal level as well as its dependence on spectral type. The level of X-ray emission increases with increasing rotation rate up to a saturation level. The rotational period at which saturation occurs is colour-dependent and increases with advancing spectral type. Also the L X / L bol – P and L X / L bol – R 0 relations are fitted with linear, exponential and power laws to look for the best fitting function. Among the fastest rotating stars (